The overall objective of our grant is to define the role that genetic and environmental factors play on the survival time of x-irradiated mice undergoing graft vs host (GVH) after reconstitution with H-2 disparite bone marrow cells. One aim was to establish the number of independently segregating genes which controlled products previously assigned to the presumed single gene, MLS. Clarification of the ambiguities of Mls is an important point for our work related to the suspected ability of Mls products to modulate GVH induced by H-2 disparite cells.